Camptothecin is a water insoluble, optically active alkaloid obtained from the Camptotheca acuminata tree. 20(S)-camptothecin and 20(S)-camptothecin analogs are cytotoxic agents that are thought to act by stabilizing a topoisomerase I-induced single strand break in the phosphodiester backbone of DNA, thereby preventing religation. This leads to the production of a double-strand DNA break during replication, which results in apoptosis if not repaired.
20(S)-camptothecin and many 20(S)-camptothecin analogs are water insoluble. Many of these drugs exhibit excellent antitumor activity against human cancer cell lines and in vivo animal xenografts. However, their water insolubility makes it difficult to administer these drugs. Additionally, the pharmacologically important lactone ring of 20(S)-camptothecin and its analogs is unstable in the presence of human plasma albumin which results in the conversion of the active drug to the inactive carboxylate form which is bound to the albumin.
One approach to overcome the pharmaceutical and pharmacokinetic shortcomings of 20(S)-camptothecin and 20(S)-camptothecin analogs is to covalently bind them to neutral polymers such as polyethylene glycol (see, e.g., references 1 and 2 below). Using this approach, the water solubility of the most active camptothecins can be improved such that the conjugated polymers can be parenterally administered in aqueous medium.
There is a continuing need for new polymeric conjugates that are capable of solubilizing a greater amount of 20(S)-camptothecin or 20(S)-camptothecin analog per polymer chain to decrease the total mass of polymer needed for administering a given dose of the active drug. As well, there is a continuing need for new polymeric conjugates that may have unique properties as antitumor agents that are not found in unconjugated water-soluble prodrugs and analogs of 20(S)-camptothecin.